The computer industry has continuously introduced various multi-media computers, many of which have audio signal input/output capability together with animated image display capability. Such a multi-media computer generally employs a multi-window display system to display different information simultaneously and independently on different areas called windows resulting from splitting a display screen, and plural icons are usually displayed so that an operator can activate any one of them uslng a device called a mouse.
In a conventional multi-window display system, a window, in which information that is not currently referred to by an operator is being displayed, can be closed to display an alternative to that window. i.e., an icon image with a smaller area. Thus, the area of a screen is effectively used. When closing an unnecessary window to display it as an icon image, a pattern to be applied to an icon image is read from a corresponding memory for display. Thereafter, such an icon image is displayed as a still image keeping the same pattern until it is activated to open a window.
There are a certain type of icon images capable of changing their patterns whilst being displayed on the screen of a display unit. For example, when an icon image is selected by an operator with a mouse for initiating input operations, such an icon image changes its pattern so as to indicate its current status of "being selected". For the case of an icon image of a clock, such an icon image can change its pattern so as to show time. For the case of an icon image indicative of whether an electronic mail is received, it can change its pattern when a monitoring system detects the receipt of an electronic mail. However, it will take at least a period of some several seconds for an icon image to be able to change its pattern. Any shorter period than that results in disadvantageously increasing the burden to be handled by a CPU. This prevents conventional techniques from producing an icon image which shows no awkward movements.
There are several drawbacks presented by conventional multi-media computers.
For example, with a first piece of information being displayed in a first window, and with a first audio signal corresponding to the first piece of information being output through a loudspeaker in the form of a sound, if the first window is closed and displayed as an icon image in order that a second piece of information is displayed in a second window, both the first and second audio signals will be output in the form of a sound at the same time. This causes hearing confusion to an operator.
Meanwhile, if an audio signal corresponding to information of a closed window in the form of an icon image is stopped, that is to say, if such an audio signal is not output currently through a loudspeaker in the form of a sound, this allows an operator to clearly hear a sound corresponding to information displayed in a new-opened window. However, this presents a problem that it is most hard for an operator to visually realize theft there exists a concealed sound behind a closed window now in the form of an icon image, since such a closed window is displayed as a still image with no movements.
Recently, user-friendly information processors such as personal computers and workstations have been developed. Several windows are opened at the same time on a display screen so that different information items can be displayed in such opened windows. Additionally, in some information processors, video signals are input for live-video image display in a window.
In conventional information processors, for example, if two different windows are opened at a time to create an overlapping field between the windows, one of these two windows becomes a foreground scene so that its entire information can be displayed without any "wane", while the other window becomes a background scene so that its entire information cannot be displayed. In other words, the background scene window is partly hidden by the foreground scene window. The amount of displayable information on a display screen is restricted by the screen size. The operator must change the foreground-to-background scene hierarchically relationship between overlapping windows or drag an obstructive window from over a target window if the operator wants to view information hidden, which is troublesome.
A solution to the above-described problem may be given employing an image translucent synthetic technique used in the field of television broadcast, more specifically, by introducing such a technique into a multiwindow information processor. For example, every pixel is assigned a respective blend ratio .alpha. (0.ltoreq..alpha..ltoreq.1). Each blend ratio .alpha. is stored at and read from a blend ratio buffer. PIXEL A of a window and PIXEL B of another window are blended by a pixel blend system according to the following calculation formula: EQU .alpha..times.A+(1-.alpha.).times.B (i)
This allows a translucently synthesized image to be displayed at a window overlapping filed. Since the blend ratio can be determined for every pixel, the setting of the blend ratio can be made with more flexibility.
The above-described image blend system contains a weighted average circuit used to blend two pixels. Three different types of weighted average circuits are known in the art.
FIG. 57 shows a first type of weighted average circuit used to blend images. The first type weighted average circuit comprises two multipliers 632 and 633 each of which performs calculations according to the calculation formula (i) and an adder 634.
FIG. 58 shows a second type weighted average circuit used to blend images. The second type weighted average circuit is formed by a subtracter 635, a multiplier 636, and an adder 637 for performing calculations according to the following calculation formula as a result of changing the calculation formula (i): EQU .alpha..times.(A-B)+B (ii)
FIG. 59 shows a third type weighted average circuit used to blend images. The third type weighted average circuit is formed by an adder 638 and n selectors (only four selectors 639, 640, 641, and 642 are shown in the figure). Weights are set with respect to the adder 638 as follows. If the number of selectors is n, n weights 1/2, 1/4 . . . 1/2.sup.n are set in such a way that these n weights form a geometrical progression of a common ratio of 1/2. Each of the n selectors selects either one of input A and input B both represented by binary numbers, as a result of which n values are selected by the n selectors. Each of the n values is multiplied by an assigned weight, thereafter those products being summed for image blend.
In the above-described conventional weighted average circuit, every pixel is given a respective blend ratio. Although various types of translucently synthesized images may be set and the flexibility of image display is good, the conventional weighted average circuit, however, produces the problem that all the blend ratios assigned to pixels forming a window must be updated one by one. As a result, the updating of blend ratios cannot be carried out at high speed. This gives a poor response time when, for example, moving a window from one place to another. Such a problem becomes more serious when the number of blend ratio change steps is increased so as to smoothly perform translucent image change at the time of the change in blend ratio. The number of bits necessary for representing blend ratios by binary numbers increases. This gives a poorer response time in updating blend ratios.
The first type weighted average circuit requires two multipliers so that it becomes large in circuit size.
The second type weighted average circuit, compared with the first type weighted average circuit, employs only one multiplier so that it does not become large in circuit size. The second weighted average circuit, however, suffers from a problem. Use of a subtracter produces some points within the circuit where a negative number must be represented for transmission. At such points, arithmetic operation corresponding to the negative number is performed. This somewhat complicates circuit organization. Since the number of arithmetic operation circuit levels is great, this not only lengthens the time required for calculation (i.e., delays in signal transmission) but also decreases the speed of operation.
The third type weighted average circuit, formed by selectors and a multi-input adder, has nearly the same circuit size and operation speed as a single multiplier. The third type weighted average circuit, however, suffers from the problem that its actual output value does not preciously agree with .alpha..times.A+(1-.alpha.).times.B. The actual output value becomes K.times.{.alpha..times.A+(1-.alpha.).times.B} (K=(2.sup.n -1)/2.sup.n, slightly smaller than 1). The brightness of video signals is decreased.
If the blend ratio .alpha.=1 or the blend ratio .alpha.=0 (i.e., if the blending of plural input signals is not required), it should be sufficient to select one from among input signals and output the as-selected input signal with no calculations involved. Even in such a condition, each conventional weighted average circuit, however, performs unnecessary calculations based on the blend ratio. This increases the amount of power consumption.
In the third type weighted average circuit, if the number of bits for a blend ratio is increased and the number of bits for an adder is correspondingly increased in order to designate the value of the blend ratio a at short intervals, this may produce a problem. More specifically, when a blend ratio represented by many bits is designated, this may give rise to such a situation that another different blend ratio represented by a smaller number of bits can be used with the same blend operation effect. In such a case, unnecessary power is consumed.